Bacillus thuringiensis is a potent microbial insecticide that is widely used to control agricultural pests, and mosquitoes and blackflies which are vectors of human diseases. The active insecticidal components are the delta-endotoxins which vary in specificity, but are believed to have similar modes of action. They are processed from the protoxin form to active toxins by proteases in the insect midguts. The active toxin binds to receptors and causes cytolysis by forming a pore or ion channel. The overall goal of this proposal is to understand in detail the biochemical mechanisms of action of the protein toxins in their interaction with the receptor protein and their functions of membrane insertion and ion channel activity. The specific aims of the present proposal are to identify the amino acids involved in receptor binding and ion channel functions of insecticidal toxins. Molecular, genetic, biochemical, and electrophysiological techniques such as site-directed mutagenesis, chemical modification, voltage clamping, and patch clamping will be used to probe toxin function. Knowledge gained from this project will provide an understanding of the mode of action of insecticidal crystal proteins in order to design more specific and more potent toxins against insect pests. The long range goal is to use this information to design better biopesticides against insect vectors of human disease.